Keep in mind that converting between formats is complicated by the DRM (digital rights management). You can't convert a protected book from one format to another without breaking the DRM first. You might want to check the legality of doing that in your area.
From experience with the Kindle - if you want to use it for text-books, I'd go for the one with the keyboard on it. It's bigger, but searching for stuff on the keyboard free one is very, very slow and cumbersome. I would find text only textbooks to be fine on the ebook reader screen, but for textbooks with diagrams or figures or tables (like physics), I'd want to use a computer or larger format tablet like the Kindle instead.
As far as selection goes, I'd go on to the catalogues you would use, and search for the kinds of books you'd buy, comparing availability and prices. I think that the Kindle charges an extra international fee in some countries (like mine), so an ebook version of a paperback novel costs about 20% more on the kindle than it does in hardcopy format (or if buying in the US). [I'm not 100% sure because I can't check the US store, but on my store, a long released novel is $9.59 US in Kindle, and usually $7.99 in paperback].
I ended up buying a Kindle for the sole reason that they would sell stuff to me, which the other ebook readers wouldn't ,and it means I can easily buy English books.